Size Distribution and Anisotropy of Self-assembled MnAs Nanoparticles in GaAs

The size distribution and anisotropy of composite films of MnAs nanoparticles in a GaAs matrix have been determined by \textit{thermomagnetic} measurement and subsequently confirmed by electron microscopy studies. The composite was fabricated from a homogeneous Ga$_{0.9}$Mn$_{0.1}$As film grown by MBE and annealed at 520-570\r{ }C to produce superparamagnetic particles of diameter 10-50 nm. Magnetization measurements show a peak in the temperature-dependent zero field cooling (ZFC) moment, $m_{ZFC}$(T), near the system blocking temperature T$_{B}$. The distribution in T$_{B}$ was first obtained from $f$(T$_{B})\propto $ d/dT [T$\cdot m_{ZFC}$(T)], derived assuming that the moment of a particle below its blocking temperature is zero and varies as 1/T above the blocking temperature. The distribution in particle diameter $f$(D) was obtained using the usual relation between blocking temperature and particle volume, K$_{eff}$V/k$_{B}$T$_{B}$=25, where K$_{eff}$ is the effective MnAs anisotropy constant. A value for the anisotropy constant was obtained by comparing the thermomagnetic $f$(D) with the size distribution obtained visually from SEM micrographs, where $<$D$>$=12 nm and width $\Delta $D=7nm for a 50-nm-thick film. The visual and thermomagnetic size distribution functions are found to be nearly identical using K$_{eff}$=160,000 erg/cm$^{3}$. Work supported by NSF DMR-097007.